This invention relates to a method of polymerizing vinyl chloride monomers, and more particularly to such a method wherein polymerization scales are prevented from adhering onto the surfaces of a polymerization vessel and associated equipment used in the polymerization of vinyl chloride monomers. The term "vinyl chloride monomer" will be used herein to mean vinyl chloride used alone, or a mixture of monomers comprising a major proportion of vinyl chloride and a minor proportion of other monomers copolymerizable with vinyl chloride, unless, of course, by the context such term has another meaning.
In suspension polymerization of vinyl chloride monomer in an aqueous medium, in the presence of a dispersing agent and an oil-soluble catalyst; or in an emulsion polymerization of vinyl chloride monomer in the presence of an emulsifier and a water soluble catalyst; or in a bulk polymerization of vinyl chloride monomer, polymer scales had always formed and adhered to the inner surfaces of the polymerization vessel and associated equipment used therewith, in prior art processes.
Polymer scale formation and adherance causes many disadvantages and deficiencies, such as decrease of thermal efficiency due to decrease in thermal conductivity, decrease in yield, decrease in quality of the product formed because of the mixing of the scales which may have become separated from the inner surfaces with the product, and decrease in operating efficiency of the equipment due to the added time required to remove the scales before the polymerization of the next charge.
In the prior art, many different proposals have been made to eliminate or reduce the above deficiencies and disadvantages. However, none of the prior art has been satisfactory from a practical standpoint.
For example, one proposed prior solution, as disclosed in Japanese Pat. No. 1970-30,343, is to coat the inner surfaces with a compound comprising a nitrogen containing organic compound selected from a group of organic compounds having azo group, nitro group, nitroso group, azomethin group or azine group and amino compounds; a sulfur containing organic compounds selected from a group of organic compounds having thio-carbonyl group, thio-ether group or thio-alcohol group, quinones, ketones, aldehydes and carboxylic acids which have more than six carbon atoms. However, experiments have shown that the various practical compounds shown in the specification of this patent have only a small effect on prevention of scale formation, and hence are not satisfactory from a practical standpoint.
Another example is Japanese Pat. No. 1970-30,835 which discloses a method of coating the inner surfaces of the polymerization vessel with a dye or pigment. However, this method is not satisfactory since the dye or pigment would tend to contaminate the product.
A still further example is Japanese Unexamined Pat. No. 1973-44,375, which discloses coating the inner surfaces with an inhibitor of free radicals. Alpha methyl styrene is disclosed to have a marked effect, among various free radical inhibitors. However, in contrast to the disclosed results, the present inventors have run tests and discovered that coatings of alpha methyl styrene has no appreciable effect on prevention of scale formation and adhesion.
Moreover, in Japanese patent application Ser. No. 1975-147,863, by the present inventors, the inner surfaces are coated by using pyrogallol or hydroxyhydroquinone. But, these compounds do not always effectively prevent polymer scale formation without being infuenced by other factors, such as polymerization temperature, and order of charging of the starting material, since they are water soluble. Also, the presence of an emulsifier tended to affect the ability of pyrogallol and hydroxyquinone to prevent polymer scale adhesion.